1. Field of the Invention
The present invention relates to assignment of a time division multiplexed radio channel, and particularly to a radio communication method capable of changing the frame structure that is specified by a boundary between downward and upward TDMA channels, or by the boundary information between multiple access protocols of a random access channel and a DA-TDMA channel.
2. Description of Related Art
Conventionally, DA-TDMA (Demand Assignment-Time Division Multiple Access) is known as one of multiple access methods which establish channels as needed in channel resources shared by a base station and multiple terminal stations, as disclosed in Chapter 6.2 of Yamamoto and Kato, xe2x80x9cTDMA COMMUNICATIONSxe2x80x9d, published by the institute of electronics, information and communication engineers of Japan, for example. The DA-TDMA, which assigns channels in response to demands of the terminal stations, can make effective use of the channels having traffic fluctuations. As a transmission method, a variety of multiple access schemes are used such as a random access method allowing collision, and a collision avoidance method employing assigned channels.
FIG. 7 shows the entire configuration of a TDMA communication system. A base station 3 is connected with terminal stations 2 (2a, 2b and 2c) via radio channels. Terminal units 1 (1a, 1b and 1c) are connected to the corresponding terminal stations 2. The base station 3 manages the channel resources between it and the terminal stations 2 connected thereto, and establishes channels between them in response to channel establishment requests from the terminal stations 2. The channel resources are time-divided, and the channel establishment is carried out at a TDMA frame period consisting of a plurality of time slots.
FIG. 8 shows a TDMA frame structure disclosed in Japanese patent application laid-open No. 9-18435/1997, for example. Various types of channels are assigned to a plurality of time slots constituting a TDMA frame. Upward control channels are used when the terminal stations 2 make a request for channel establishment or release to the base station 3, and downward control channels are used when the base station 3 issues a command to perform channel establishment or release to the terminal stations 2. Downward and upward traffic channels are assigned to the terminal stations as needed, and are used for the data communications between the base station 3 and terminal stations 2. The present example employs in the downward channels the TDM (Time Division Multiplex), and in the upward control channels the DA-TDMA using the slotted ALOHA (Additive Links On-line Hawaii Area). The upward traffic channels can be divided into subframes associated with services such as ABR (Available Bit Rate), VBR (Variable Bit Rate) and CBR (Constant Bit Rate). In addition, it will be able to vary, in accordance with traffic, boundaries such as a boundary between the upward channel and downward channel, or between the control channel and traffic channel, and a boundary between the multiple access protocols such as S-ALOHA and DA-TDMA, or between subframes corresponding to various services.
FIG. 9 is a sequence diagram illustrating channel assignment disclosed in Japanese patent application laid-open No. 9-214459/1997, for example. Horizontal axes are a time axis, and the processings are carried out sequentially from the left-hand side. Blocks at the top of this figure represent TDMA frames, on the basis of which the processings are carried out. When the data to be transmitted takes place in FIG. 9, the terminal station 2 transmits assignment request information to the base station 3. Receiving the request, the base station 3 assigns the requested number of slots from among the available slots in a TDMA frame, and sends to the terminal station 2 channel assignment information obtained as a result of the assignment. The terminal station 2 begins data communications in accordance with the instructions.
FIG. 10 shows a TDMA frame structure disclosed in Japanese patent application laid-open No. 10-51406/1998, for example. In this example, upward control channels, which are used for requesting the channel establishment or release from the terminal station 2 to the base station 3, precede the downward control channels, which are used for requesting the channel establishment or release from the base station 3 to the terminal station 2. Downward and upward traffic channels are allotted between the upward and downward control channels.
FIG. 11 is a sequence diagram illustrating channel assignment, in which the horizontal axes are a time axis, and the processings are carried out sequentially from the left-hand side. Blocks at the top of this figure represent TDMA frames, on the basis of which the processings are carried out. When the data to be transmitted takes place in FIG. 11, the terminal station 2a transmits assignment request information to the base station 3. Receiving the request, the base station 3 assigns the requested number of slots from among the available slots in a TDMA frame at a downward/upward traffic channel period, and sends to the terminal station 2 channel assignment information obtained as a result of the assignment through the downward control channel of the same frame. The terminal station 2 begins data communications from the next frame in accordance with the instructions.
Finally, FIG. 12 shows a method for assigning a plurality of slots disclosed in Japanese patent application laid-open No. 8-265836/1996, for example. FIG. 12 illustrates only a single frame on a upward traffic channel, in which the frame consists of five slots designated by S1-S5. At stage 1, the slots S1, S4 and S5 are assigned to the terminal station 2a, the slot S3 is assigned to the terminal station 2b and the slot S2 is assigned to the terminal station 2c. Discontinuous slots thus assigned will result in divided transmission bursts. Generally, since an overhead like guard time or an alignment symbol is required for each transmission burst, assigning continuous slots is preferable to improve the efficiency. Stage 2 illustrates a state in which the terminal station 2c completes its call. At stage S3, the slot S2, which becomes available owing to the call completion, is reassigned to the terminal station 2b instead of the slot S3. At stage 4, continuous slots S3, S4 and S5 are reassigned to the terminal station 2a. Thus, the conventional reassignment is achieved by independently changing the assignment of the slots to the individual terminal stations 2 in accordance with the timings of originating or terminating a call.
As described above, the conventional TDMA radio communication systems achieve the channel assignment by assigning a channel to each terminal station 2, by changing the assignment and by reassigning a channel. Therefore, it is very difficult for them to consider the entire terminal stations 2 belonging to the base station 3 to carry out the channel assignment or reassignment at the same time. Thus, it is difficult for them to implement optimum channel assignment considering characteristics of the entire terminal stations and the channel assignment requests. In particular, it is not unlikely that the entire channel assignment of all the terminal stations 2 must be changed to vary in accordance with the traffic the boundary such as the boundary between the upward channel and downward channel, between the control channel and traffic channel, between the multiple access protocols, or between subframes corresponding to various services. Accordingly, it is necessary to design a TDMA radio communication system capable of changing the channel assignment of all the terminal stations 2 at the same time.
The simultaneous change of the channel assignment to all the terminal stations 2 is difficult in the conventional TDMA radio communication system because it is not rare that only insufficient reliability can be obtained of the upward and downward control channels. It is necessary for the radio communications to consider the possibility that errors will take place on the control channels because of fluctuations in communication quality due to fading or shadowing. As for the individual change of the assignment to each terminal station 2, it has no effect on the remaining terminal stations even if the reliability of the control channel is insufficient, although the channel assignment to that terminal station 2 is impossible. In addition, using a retransmission means such as ARQ (automatic request for repetition) will complete the channel assignment with some delay. In contrast, in the simultaneous change of the channel assignment to all the terminal stations, a transmission error on the control channel can cause collision between the transmission signals from the normally operating terminal station 2, to which the current slot is assigned rightly, and from the terminal station 2 which carries out erroneous transmission, thereby resulting in transmission error of the traffic channels. Therefore, it is necessary to design a TDMA radio communication system that can achieve transmission with maintaining channel assignment information and changing timing for all the terminal stations 2 with sufficient reliability until the changing timing has passed.
The reliability of the control channel can be improved by introducing error correcting code or by some error control technique like retransmission of the same information. However, improving the reliability of the control channel by retransmission will take extra time for the transmission on the control channel, resulting in delay in call connections. It is preferable that the delay of connections is as small as possible because it causes degradation of the service and an increase of the buffer capacity of communication data. Accordingly, it is necessary to design a TDMA radio communication system capable of reducing the channel assignment time as small as possible so far as this does not affect the remaining communications.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a TDMA radio communication method capable of achieving the simultaneous channel assignment to all the terminal stations connected to a base station with small delay and high reliability.
According to a first aspect of the present invention, there is provided a TDMA radio communication method which has super frames each consisting of a plurality of frames, and which includes in the frame a control channel for exchanging control information between a base station and at least one terminal station, the TDMA radio communication method comprising the steps of: providing a latter part of each of the super frames with an assignment information notification period which is preceded by an assignment processing period, and providing a remaining part of the each of the super frame with an assignment request collecting period; transmitting, from the terminal station to the base station, assignment request information using a plurality of frames included in the assignment request collecting period; transmitting, from the base station to the terminal station, frame structure information and assignment information over a plurality of frames included in the assignment information notification period; and carrying out, in the base station, channel assignment of a radio channel, changes of the frame structure and of the channel assignment in response to timings of the super frames.
According to a second aspect of the present invention, there is provided a TDMA radio communication method which has super frames each consisting of a plurality of frames, and which includes in the frame a control channel for exchanging control information between a base station and at least one terminal station, the TDMA radio communication method comprising the steps of: providing a latter part of each of the super frames with an assignment information notification period which is preceded by an assignment processing period, and providing a remaining part of the each of the super frame with an assignment request collecting period; transmitting, from the terminal station to the base station, assignment request information; carrying out, in the base station, channel assignment from among available slots in a current super frame in response to the assignment request information received by the base station during the assignment request collecting period, and transmitting information on the channel assignment to the terminal station; carrying out, in the base station, channel assignment from among available slots in both the current super frame and next super frame in response to the assignment request information received by the base station during one of the assignment processing period and the assignment information notification period, and transmitting information on the channel assignment to the terminal station; transmitting, from the base station to the terminal station, frame structure information and assignment information over a plurality of frames included in the assignment information notification period; and carrying out, in the base station, changes of the frame structure and of the channel assignment in response to timings of the super frames.
Here, the TDMA radio communication method may further comprise the step of transmitting the assignment request information and the assignment information a plurality of times to make a majority decision at a receiving side.
The frame structure information may comprise information on a super frame length and a frame length, and one of two pieces of boundary information consisting of boundary information between an upward channel and a downward channel, and boundary information between protocols of multiple access channels such as a random access channel and a TDMA channel, and the TDMA radio communication method may further comprise the step of carrying out optimum channel assignment using the frame structure information.
The TDMA radio communication method may further comprise the step of changing the channel assignment to enable the terminal station, to which discontinuous slots are assigned in a current super frame, to use consecutive slots in a next super frame.
The TDMA radio communication method may further comprise the step of inhibiting transmission of a next super frame from the terminal station which cannot receive the assignment information during the assignment information notification period, until the terminal station receives the channel assignment information.